This invention relates to the guiding and cooling of casting belts in apparatus and procedure where metal is continuously cast on such belts, i.e. between endless belts, arranged to discharge solidified metal strip. In a notably important sense, the invention is concerned with improvements for methods and machines for casting metals, such as aluminum (including aluminum alloys) and zinc, and other metals, which melt at moderate and low temperatures, between a pair of moving surfaces conveniently constituted of flexible heat-conducting bands or belts that have conventionally been metal belts in twin-belt casters of this sort.
In a specific sense, the invention is related to belt casting apparatus and procedure such as described in U.S. Pat. Nos. 4,061,177 (Sivilotti) and 4,061,178 (Sivilotti, Steer and Stock), both issued Dec. 6, 1977. The structures and operations set forth by way of example hereinbelow and in the accompanying drawings will be shown and described as if embodied or substituted in the machine of the above patents, and such patents are therefore incorporated herein by reference, especially for the sake of any further disclosure that may be deemed desirable to facilitate further understanding of the present invention.
As will be understood, continuous metal strip casting of the sort identified above, involves the use of a pair of endless belts, usually made of flexible but stiffly resilient steel or the like, which are driven over the appropriate rollers and other path-defining means, so that they travel together along a space, usually downward-sloping or horizontal, which represents a mold region. Molten metal is introduced between the belts in the vicinity of the upstream entry end of such region, and the metal is discharged as solidified strip, from the downstream exit end of the space. In accordance with the disclosures of the above U.S. patents, means are provided for guiding and cooling the belts as they traverse their paths which define the mold space. Such means may consist of a multiplicity of guide elements or nozzles, collectively covering the entire area slightly rearwardly of the reverse surface of each belt. Liquid coolant, i.e. water, is projected through each of such nozzles, which have a substantially flat face, with the result that flowing water continuously covers the major part of the reverse or outer belt surface. Indeed, the layer of water may serve as the bearing for the belt. In the described apparatus, a liquid-bearing arrangement is also employed, where the belts are returned over a curved path to re-enter the mold space. Although it is conceivable that the improvements described below may be used in part or all of the curved guide means for such liquid bearing, the new structures are of special advantage for guiding the belts along their substantially plane paths defining the mold space, especially where some compliance of the guiding elements is needed, i.e. to move outwardly very slightly through force exerted on the belt at certain stages of the casting operation.
In the arrangements of the above patents, each casting belt is cooled and guided by a collective assembly of the above-mentioned nozzles, which have had circular faces, arranged in a close-fitting pattern at a slight distance from the reverse surface of the belt.
As will be understood, coolant liquid is projected through the central jet openings of the nozzles and flows rapidly between the nozzle face and the belt, forming the desired cooling and belt-supporting layer of the liquid. The liquid returns through the triangle-like space between adjacent nozzle faces. The effect is generally to provide a continuous layer of rapidly flowing coolant next to the belt, but although the apparatus operates usefully with the foregoing arrangement, some difficulty has been experienced with the existence of less cooling in the above-mentioned triangular areas that are employed for drainage (and that also provide access for releasing certain parts to permit removal of the nozzles) and there have correspondingly been some metallurgical defects in the cast strip, believed to result from the localized, excessive belt temperatures. In other words, these localized temperatures are found to exist and appear to have some adverse effect even though the temperature rise is relatively brief or small, since it is governed by the intermittent passage of the belt over the particular drainage areas which represent not more than about 10% of the total belt surface. In any event, although the machines of the above patents are eminently useful, some room for improvement in the above respects has been observed.
It has now been found that the requirements of belt guiding and cooling involve underlying structure for a very thin and very rapidly flowing layer of liquid coolant, with as little unsupported area as possible, i.e. with minimization of the space or spaces needed for draining the coolant liquid. Furthermore, to the extent that the belt cooling and guiding faces are separate elements, as is desired for the individual compliance described in the above patents, provision must be made for ready removal and replacement of such elements, i.e. for normal maintanence against wear or other deterioration; as will be understood, it is critical for surfaces constituted collectively by such units, for a high degree of dimensional and positional accuracy, as well as being characterized by a controlled degree of compliance where needed.